This invention relates generally to the use of extrusion coating techniques for forming multilayer paint coated films. More particularly, multilayer coatings are made by extrusion coating one or more layers onto a surface-controlled plastic carrier sheet to produce high quality films at high speeds while avoiding solvent emission problems characteristic of the use of solvent-based paints.
1. Exterior Automotive Applications
Exterior automotive body panels have been made in the past by spray painting sheet metal parts. Multilayer paint coats, such as those referred to as a clear coat/color coat paint finish, have been used to produce desirable optical effects. In addition to high gloss and high distinctness-of-image (DOI), these paint coats also are highly durable by providing chemical resistance, abrasion resistance and weatherability that avoids degradation by ultraviolet light.
In more recent years molded plastic car body panels have been made with decorative clear coat/color coat paint films bonded to the molded plastic panel. Use of such films avoids certain environmental problems associated with evaporation of paint solvents while also reducing or eliminating the need for paint facilities and emission controls at the automotive production plant.
Because of the growing need to reduce the amount of atmospheric pollution caused by solvents emitted during the painting process, many different approaches have been taken in recent years for producing these decorative films. These processes are generally categorized by solution casting techniques or extrusion techniques. For instance, U.S. Pat. Nos. 4,810,540 to Ellison et al., and 4,902,557 to Rohrbacher use solution casting techniques in which liquid-cast, solvent-based clear coats and pigmented base coats are applied to a flexible casting sheet by a coating process such as reverse roll coating or gravure printing. The liquid cast layers are separately applied and then dried at high temperatures to evaporate the solvents.
As an alternative, extruded films have been used for making exterior automotive clear coat/color coat films. International application PCT US93 07097 to Duhme describes an approach in which paint films are made by coextruding a base coat and a clear coat as separate extruded layers onto a carrier sheet. The carrier is used as a supporting sheet for the clear coat and color coat in a subsequent injection molding process. The extruded clear coat and color coat are supported in the mold by the carrier and shaped in the mold. The clear coat is a coextruded sheet having different proportions of polyvinylidene fluoride (PVDF) and acrylic resins in each layer of the coextrusion.
U.S. Pat. Nos. 4,317,860 and 4,364,886 to Strassel also disclose coextrusion of multilayer films such as a two-layer coextrusion of predominantly PVDF on one side and a predominantly acrylic resin on the other side of the extruded sheet. These unitary structures are used to make molded articles, or to adhere the sheets to a molded polymer.
Film extrusion techniques also have been used in the past for making free films in which the extruded polymeric material is coated on a polished drum. These films are then undercoated with various color coats. The exterior surface of the extruded free film that contacts the drum (and is separated from the drum as a free film) does not have a high gloss and distinctness-of-image. Also films manufactured in this manner do not have a carrier sheet attached, which makes them hard to handle and easily damaged in subsequent processing.
Another approach disclosed in U.S. Pat. No. 5,114,789 to Reafler comprises a pigmented base coat which is solvent-die extrusion coated onto a flexible, stretchable carrier sheet and dried at elevated temperatures to evaporate solvents, followed by extrusion coating a reactive clear coat on the base coat. The carrier film and extrusion coated paint layers are then heat softened as a unitary sheet and applied to a molded shaped substrate by a shrink wrap process.
In a currently used process for making exterior automotive films, a clear coat and color coat comprising blends of PVDF and acrylic resins are cast by reverse roll coater, either by solution or dispersion casting. The film thickness of the paint coats used in the process generally is dictated by end user requirements. In some instances the need to produce relatively thick films can impose certain production constraints. To adequately dry the material and to prevent air entrapment, line speeds are typically at 25 feet per minute. This slow throughput limits the coating capacity of the reverse roll coater and also releases a large amount of organic solvents. This solvent release is particularly evident when a solution-cast PVDF/acrylic clear coat is coated from a solvent-based solution having a relatively high amount of solvent. VOC emissions are high. PVDF has limited solubility and requires strong solvents to dissolve. One such solvent known as N-methyl pyrrolidone (trade name M-Pyrol) is needed to solubilize the resin in solution casting or can be used as a coalescing aid in dispersion casting. In addition, cross contamination can occur from solubilizing residual material in previously used drums, hoses, pans, pumps, etc. Also, during coating, the strong solvent can dissolve caked-on resins in a drying oven, causing them to cascade down on the web being coated. As a further concern, these strong solvents are expensive.
Thus, there is a need for producing decorative and protective surfacing films while avoiding the adverse effects of low production line speed, high VOC, cross-contamination, and the use of expensive solvents. Extrusion techniques can be an alternative that avoids the use of strong solvents and their related solvent emission problems. Extrusion techniques such as those described above, however, have not been successfully adapted to producing high quality films at high line speeds and at low cost.
The present invention provides an extrusion coating process that is an alternative to solution or dispersion casting of polymeric films and conventional extrusion. The invention is particularly useful with PVDF/acrylic films while avoiding use of the strong solvents and their related problems described previously. Use of the extrusion coating techniques of this invention provides the advantages of avoiding expensive solvents, producing no VOC emissions, and avoiding cross-contamination associated with solvent casting. In addition, and as emphasized in more detail below, the present invention has the added advantage of greatly increasing line speed, eliminating steps in the manufacturing process, and reducing costs for producing the clear coat/color coat films.
Further, the invention has particular applicability to the manufacture of molded plastic exterior automotive body panels and parts. The invention provides a means for producing high gloss, high DOI clear coat/color coat paint films of exterior automotive quality.
2. Exterior Siding Panel Applications
Although the invention is described above with respect to exterior automotive applications, the invention also has applicability as a protective and decorative coating for other articles such as interior automotive components, exterior siding panels and related outdoor construction products, marine products, signage, and other interior or exterior film products with similar constructions.
The following description relating to vinyl (PVC) siding panels is an example of one use of the invention for producing outdoor weatherable decorative surfaces on extruded plastic sheets. The invention, however, is applicable to plastic substrate panels other than vinyl.
Wood, metal and vinyl are materials commonly used as boards or shingles for siding in the construction and remodeling of commercial and residential structures. Painted wood is perhaps the most aesthetically pleasing of these materials to the consumer, but wood suffers from deterioration by rotting and attack by insects, rodents and birds. Wood surfaces need constant re-protection with paint, stains and water sealants.
Advantages of metal over wood include the ability to provide a baked-on finish that is more resistant to the elements than coatings applied to wood, thus eliminating subsequent material and labor costs to the consumer. However, metal-based siding has less thermal insulation value and is sensitive to scratching and denting by hail and other blows to the surface. In addition, it does not have the pleasing look of a wood surface.
Vinyl siding has an economic advantage over most high quality wood and metal siding. It has better insulating qualities than metal, and it is more resistant to denting and scratching. However, vinyl is sensitive to degradation from ultraviolet rays of the sun. Previously, vinyl siding has suffered aesthetically compared to wood because of the generally higher gloss or uniform sheen of vinyl siding, which has a rather unnatural or xe2x80x9cplasticxe2x80x9d look. In addition, it is difficult to decorate vinyl surfaces with a natural-looking wood grain pattern that is pleasing to the consumer.
The present invention overcomes the drawbacks of presently available vinyl siding by providing a process that economically produces a vinyl-based siding panel with a superior outdoor weatherable surface having the look of either a painted wood surface or natural unpainted wood grain.
A vinyl siding presently on the market is made by a process in which an extruded sheet of PVC is decorated and embossed to produce a wood grain appearance. A silicone-coated release paper is used in this process to transfer a printed wood grain pattern to the vinyl. Typically, an acrylic clear coat and a color coat with a wood grain pattern are coated on the paper carrier sheet and then transfer-laminated onto the vinyl extrusion under heat and pressure. The wood grain color coat can simulate the appearance of painted or unpainted wood. The clear coat and wood grain color coat bond to the extrusion and release from the paper carrier sheet so that the clear coat forms a protective outer coating for the underlying wood grain color coat. During the transfer process, the release paper is pressed into contact with the extrusion by an embossing roll that transfers deep embossments to the surface of the transferred decorative wood grain. The deep embossments or indentations produce a xe2x80x9cshadowxe2x80x9d effect in the finished surface that more truly resembles the sheen of natural wood. Without these embossments, the vinyl wood grain-simulating surface has a flat appearance at different light anglesxe2x80x94an unnatural look that gives the unsatisfactory appearance of a plastic simulated wood panel.
There are several drawbacks to this prior art process of making vinyl wood grain panels. A principal disadvantage is that gloss levels produced by this process are too high. Surface gloss can be measured by various techniques; and according to one technique (ASTM D 3679-86, 5.11), surface gloss levels produced by the prior art process generally have a 75xc2x0 gloss reading from about 40 to about 50 gloss units. A much lower matte surface with a gloss reading below about 20 gloss units, and preferably below about 12 gloss units, is desirable to produce a more natural looking simulated wood grain finish.
In addition to its surface gloss problem, the silicone-coated paper carrier sheet of the prior art can prevent carefully controlled printing of the decorative coatings. The carrier sheet also should release freely from the decorative layer transferred to the extruded vinyl sheet. Transfer of the decorative wood grain layer to the vinyl sheet takes place at high temperatures, since the decorative coating may be transferred to the heated vinyl extrusion immediately after it exits the extruder die opening. If the vinyl sheet is embossed when the wood grain coat is transferred, embossing temperatures should be high in order to physically form the embossments. A substantial reduction in temperature of the vinyl extrusion prior to embossing can inhibit forming deep embossments. If the transferred wood grain coat has a highly roughened surface, in order to produce lower gloss, the surface roughness can interfere with proper release of the carrier sheet. At the high transfer and embossing temperatures, certain coatings on the carrier sheet can preferentially adhere to the carrier rather than properly releasing from it.
The decorative coatings transferred to a vinyl sheet to provide a wood grain or other surface finish also should protect the underlying vinyl panel from UV degradation, delamination, and other weather problems.
Thus, there is a need for a process for manufacturing decorative wood grain vinyl sheets with a sufficiently low gloss surface to resemble the appearance of natural wood grain, painted or unpainted. In a transfer process in which the wood grain coating is printed on a carrier and transferred from the carrier to a vinyl extrusion, while the vinyl is being deeply embossed, there is a need to ensure that the decorative wood grain coat properly releases from the carrier and bonds to the vinyl. The deep embossing step must not be adversely affected; the low matte surface of the decorative wood grain must not adversely affect proper release from the carrier; and these steps must be carried out at the high temperatures and pressures necessary for proper embossing.
These technical problems have been solved, to a great extent, by the process disclosed in the applicant""s U.S. Pat. No. 5,203,941 to Spain et al., which is incorporated herein by reference. The Spain et al. patent discloses solvent-cast paint films applied as a decorative coating to the matte release carrier sheet. The present invention provides a process for making the vinyl siding panels in which the disadvantages of using solvent-cast films for the outer protective coat are avoided.
One aspect of this invention provides a process for making decorative wood grain extruded vinyl sheeting suitable for outdoor use, in which the surface of the decorative wood grain has a sufficiently low gloss to resemble natural wood grain. As an option, the decorative wood grain surface can be deeply embossed to produce a natural looking wood grain surface. The low gloss wood grain coat transfers to an extruded vinyl sheet and properly bonds to the vinyl extrusion, while the carrier on which it is coated properly releases from the transferred decorative surface at elevated temperatures. The invention also provides extruded plastic siding panels with weatherable decorative surface coatings that inhibit long term UV deterioration and delamination problems.
Briefly, one embodiment of this invention comprises a process for making a protective and decorative surfacing film comprising extrusion coating a solid polymer from an extruder die directly onto a moving carrier sheet to form an extruded coating of uniform film thickness on a smooth surface of the carrier sheet. The extrusion coated layer is preferably formed as an optically clear first layer on the carrier which travels at high speed past the extruder die opening. The extrusion coated first layer is hardened by a temperature reduction, such as by contact with chill rolls, followed by applying a pigmented second layer in thin film form on the cooled first layer to form a composite paint coat in which the first and second layers are bonded to each other. An optional size or adhesive coat may be applied to the second layer. The composite paint coat is preferably transferred to a backing sheet or other substrate panel, after which the carrier sheet is separated from the resulting laminate to expose an outer surface of the first layer as a high gloss surface with a high distinctness-of-image providing a transparent protective outer coat for the pigmented second layer and the underlying substrate panel.
Although various polymeric film-forming materials can be used for forming the extrusion coated outer layer, the preferred extrudable material is a blend or alloy of a fluoropolymer and an acrylic resin in which the fluoropolymer is preferably polyvinylidene fluoride (PVDF).
The pigmented second layer, in one embodiment, can be solvent cast onto the extrusion-coated first layer, or alternatively, the first and second layers can be formed as a coextrusion which is then coated onto the moving carrier sheet.
Other forms of the invention include coextruding various layers of the composite laminate including not only the clear coat and underlying color coat but also size, tie and other functional coats as well. The carrier can also be extruded in tandem.
Since one or more layers of the composite paint coat can be extrusion coated using solid polymers, the process avoids the use of expensive solvents and also avoids VOC emissions and cross-contaminations associated with solvent casting. The process also can reduce production time and costs. A line speed for extrusion coating can be at least 50 feet per minute and in some instances in excess of 200 feet per minute, as compared to 25 feet per minute for solvent casting techniques.
Such improvements in line speed and related improvements in quality of the extrudate are produced by controlling the compatibility of the blended polymeric materials that comprise the backbone of the extruded material. By matching the melt viscosities of the blended polymeric materials in that they are reasonably close to each other, the flow characteristics of the alloyed material when heated to the extrusion temperature produce a smooth, more uniform flow which also avoids stress formation and visual defects in the hardened film.